This application relates to fabricating a device for use in detecting or imaging electromagnetic radiation, such as photons of light.
Many scientific endeavors, especially space exploration and astronomical measurement applications, require highly sensitive electromagnetic radiation detectors, such as photon detectors. Charge-coupled detectors (CCDs) provide high quantum efficiency, broad spectral response, low readout noise, and high resolution. Therefore, CCD devices have been used extensively in scientific applications.
To be effective for most scientific applications, CCD devices must exhibit nearly perfect charge-transfer efficiency. Therefore, CCD devices are fabricated using specialized processes, such as the buried channel and peristaltic fabrication processes, which leave little, if any, imperfections in the semiconductor materials from which the CCD devices are formed. The CCD devices produced by these processes are generally characterized by high power consumption.
Other imaging devices, such as charge injection devices (CIDs) and active pixel sensors (APSs), are formed using conventional complementary metal-oxide semiconductor (CMOS) processes. CMOS devices typically exhibit much lower power consumption than CCD devices. Moreover, CMOS fabrication processes allow other imaging components, such as signal-processing circuitry, to be formed on the detector chip. As a result, CMOS imagers are preferred over CCDs in some scientific applications. The use of CMOS imagers has been limited, however, by characteristically low quantum efficiencies and high levels of fixed pattern noise in captured images.
In general, the fabrication processes used to produce CCD and CMOS imagers are incompatible. Conventional CMOS fabrication processes occur, at least in part, at temperatures that produce imperfections in the underlying semiconductor materials. While generally acceptable in CMOS devices, these imperfections typically reduce the efficiency of CCD devices to unacceptable levels.
The techniques described here combine benefits of CCD and CMOS devices without integrating CCD and CMOS fabrication process technologies. The resulting detectors and imagers exhibit the performance benefits of CCD devices, including high quantum efficiency, high fill factor, broad spectral response, and very low noise levels, as well as the low power consumption and ease of component integration associated with CMOS devices.
Because the CCD and CMOS portions of an imaging device are manufactured in separate processes using separate substrates, the CCD and CMOS portions can be tuned separately for optimum noise performance. Moreover, fabricating the CCD and CMOS components from separate substrates allows the use of backside thinning and illumination techniques, as well as backside passivation techniques such as delta doping.
In some aspects, the invention features techniques for fabricating a radiation detector, such as an imaging device. An array of detectors, such as charged-coupled devices, are formed on a first substrate using a CCD fabrication process, such as a buried channel or peristaltic process. One or more charge-converting amplifiers are formed on a second substrate, typically using a CMOS fabrication process. The two substrates are then bonded together to form a hybrid detector.
Other embodiments and advantages will become apparent from the following description and from the claims.